Vapor-liquid contacting devices are used in a wide variety of applications for separating liquid or vapor mixtures. One of the major applications of the vapor-liquid contacting devices is in the separation of chemical compounds via fractional distillation. These devices are also used to contact a gas stream with a treating liquid which selectively removes a product compound or an impurity from the gas stream.
Within a column containing vapor-liquid contacting devices, liquid flows in a generally downward direction and vapor rises vertically through the column. On each vapor-liquid contacting device, liquid flows in a generally horizontal direction across the device and vapor flows up through perforations on the device. The cross flow of vapor and liquid streams on each device generates a froth for intimate vapor-liquid contacting and mass transfer.
The apparatus can be used in the separation of essentially any chemical compound amenable to separation or purification by fractional distillation. Fractionation trays are widely used in the separation of specific hydrocarbons such as propane and propylene or benzene and toluene or in the separation of various hydrocarbon fractions such as LPG (liquefied petroleum gas), naphtha or kerosene. The chemical compounds separated with the subject apparatus are not limited to hydrocarbons but may include any compound having sufficient volatility and temperature stability to be separated by fractional distillation. Examples of these materials are acetic acid, water, acetone, acetylene, styrene acrylonitrile, butadiene, cresol, xylene, chlorobenzenes, ethylene, ethane, propane, propylene, xylenols, vinyl acetate, phenol, iso and normal butane, butylenes, pentanes, heptanes, hexanes, halogenated hydrocarbons, aldehydes, ethers such as MTBE and TAME, and alcohols including tertiary butyl alcohol and isopropyl alcohol.
One important issue in the field of vapor-liquid contacting columns is improving the capacity and mass transfer efficiency of the trays. By improving capacity and mass transfer efficiency smaller columns may be used or less trays may be needed to achieve a similar result.
Supports are necessary to keep the vapor-liquid contacting tray in position. However, the perforations on the vapor-liquid contacting trays above the support are blocked by the support and no vapor can pass through these areas. Therefore, the zones above the support become dead zones in which little or no vapor-liquid contact occurs, reducing the active area of the tray. Tray capacity is reduced when a significant amount of active tray area is blocked by support means. Tray efficiency is also decreased since little vapor-liquid contact occurs in the dead zones.
Within a vapor-liquid contacting column, a support ring is attached to the inside perimeter of the column wall to provide support for a vapor-liquid contacting tray. The vapor-liquid contacting tray generally rests on the support ring, and the support ring creates a dead zone around the perimeter of the vapor-liquid contacting tray. Additionally, for large trays a support beam, such as a flange or I-beam, is attached to the support ring or column wall, and may be utilized to provide additional support for a vapor-liquid contacting tray. Similarly, the area of the vapor-liquid contacting tray directly above the support beam becomes a dead zone due to the blockage of vapor flow to that particular area of the vapor-liquid contacting tray.
Traditionally vapor-liquid contacting trays are affixed to a tray support ring for support. The tray support ring blocks a large amount of the active area of the tray. A vapor-liquid contacting column with an internal diameter of 2.44 meters (8 feet) possesses an internal cross-sectional area of about 4.68 square meters (50 square feet). The tray support ring with a width of 7.62 cm (3 inches) reduces the active cross-sectional area to about 4.10 square meters (44 square feet), a reduction of about 12%. This reduction does not include other contributors to dead zones such as beam supports, downcomer angles, splice angles or downcomer end support plates.
To reduce the amount of dead zone area, holes may be manually drilled into the support means to activate a portion of the dead zone. However, this method is inconsistent with the initial design of the vapor-liquid contacting tray as the manually drilled holes often vary in size, angle, pitch and location. Additionally, vapor-liquid contacting trays may have shaped holes or louvers as to accelerate liquid flow, or control vapor flow in a particular manner that manually drilled holes would not replicate. Furthermore, drilling holes into the support compromises the integrity of the support, potentially creating a need for additional support means.
U.S. Pat. No. 6,799,752 issued to Wu et al. on Oct. 5, 2004, discloses a method for activating tray dead zones such as those above major support beams by installing microdispersers on the dead zones. However, these microdispersers may interfere with liquid flow when they are installed perpendicular to the liquid flow. The microdispersers may also reduce available space for liquid/froth flow across the microdispersers when the support beams are aligned vertically one above another.
Therefore, an apparatus that can adequately support vapor-liquid contacting trays while increasing the active cross-sectional area of the tray by reducing dead zones without impeding liquid flow is desired.